Cast iron



May 22, 1962 H. H. KESSLER ETAL 3,035,911

CAST IRON Filed July 29, 1960 WEDGE Mg VALUE ANALYSIS 5x WHITE .20 E

Y ZONE 0F CARBIDE STABILITY N D D ZXWHITE .09-

ZONE OF INCREASING CARBIDE STABILITY AND DECREAING CARBIDE \QAETASTABILITY Ix WHITE .06 C l 7 ZONE OF CARBIDE o I B METASTABILITY MOTTLED .04

'-7 v ZONE OF CARBIDE INSTABILITY 0R GRAPHITE STABILITY GRAY A AI CAR BlDE METASTABILIZING ADDITION CARBIDE DESTABILIZING ADDITION INVENTORS HARRY H. KESSLER WILLIAM H. MOORE A TTORNE YS United States atent 3,035,911 CAST IRON Harry H. Kessler, 7 Dromara Road, Clayton, Mo., and William H. Moore, 19 Villa Road, Larchmont, N.Y. Filed July 29, 1960, Ser. No. 46,278 7 Claims. (Cl. 75130) This invention relates to cast iron practice in general, and specifically to a superior method of making nodular cast iron. The cast iron may be hypoeutectic or hypereutectic in composition, but is one in which the uncombined carbon appears in the nodular or spheroidal form. This form of carbon is produced by the presence of carbide metastabilizing agents in the melt.

The present invention is directed to an improved process of making cast iron with conventional carbide metastabilizing agents such as magnesium and cerium. As will be appreciated, alloys of these materials may be readily used. The process comprises adding a special agent after the carbide metastabilizing agent or nodule impelling agent has been incorporated into the melt.

For the sake of clarity, magnesium will be used as the example of such an agent. Magnesium, when present in cast iron in excess amounts, detracts from the ductility of the as-cast material by the formation of excessive quantities of carbides. Most commercial castings containing more than 0.04% magnesium have to be annealed in order to provide complete freedom from carbides. Magnesium also combines very readily with the oxygen in the air with the formation of oxide dross. This dross leads to well-known casting defects when it becomes entrained in the metal. The amount of dross produced is in direct relation to the amount of magnesium in the iron.

Magnesium in the cast iron in amounts in excess of .G4% also has a serious effect on the soundness of the castings. This is probably because the vapor pressure of magnesium tends to prevent complete solidification of the crystal dendrites. In commercial practice those skilled in the art generally always try to keep the magnesium content to the barest minimum necessary for nodularity. This is the range of about .04%. Unfortunately, this practice does not allow for the fading effect due to loss of magnesium effect, and does not insure full modularity in slower cooling heavy casting sections.

It is the object of this invention to provide a cast iron 1n which the graphite substantially exists in the combined nodular and spherulitic form in the as-cast condition, and in which the magnesium content is sufficiently low to insure relative freedom from founding defects associated with high magnesium contents.

Another object of the invention is to produce a melt wherein all the carbides are metastable by adding sufficient carbide metastabilizing agents to produce stable carbides and then rendering the stable carbides metastable by adding the carbide destabilizing agent of the present invention.

It is a further object to provide a nodular cast iron of improved castability and shrinkage characteristics free from hard edges, pinholes and dross defects.

A still further object is to provide a means of removing excess carbide metastabilizing agents from the melt so as to provide uniform and dependable results. Other objects and advantages of the invention become apparent to those skilled in the art from the following description:

It is known by those skilled in the art that nodular cast iron may be produced by adding certain agents to the melt. Because of the Whitening effect of these agents, the second step of conventional procedures is to add a graphitizing agent to the melt. This normally consists ice of ferro silicon or other silicon alloys which have a powerful graphitizing efiect. In many cases, certain fluxes are also added to the melt as a final step in order to remove dross, dirt, and other contaminants which may be present.

The present invention differs from the well-known method of producing cast irons having nodular and spherulitic forms of graphite, in that a special agent is used which produces a carbide destabilizing efiect by reducing the efiective carbide metastabilizing agent content in the melt, usually to a value of below that normally required for modularity. This destabilizing efiect renders graphitization of the melt more positive, and reduces the danger due to the presence of excess stabilizers in the melt.

This concept of destabilizing the carbide forming propensity of nodular impelling agents has not previously been disclosed. It has always been theorized that the presence of carbide stabilizers such as magnesium, used to initiate the formation of nodular graphite, are essential to the process of nodularization. Thus all efforts have been directed towards maximum recovery of magnesium from alloy additions and towards maintaining a concentration of magnesium in the melt above the critical minimum quantity supposedly necessary to insure nodular graphite. Any harmful effects due to the presence of this critical quantity of magnesium have always been regarded as a necessary evil.

The principle of this invention is best illustrated by the diagram shown in the drawing. Starting oif with carbon in the form of graphite in the melt and adding metastabilizing agents to the melt of the nodular impelling type produces a zone of carbide metastability. This occurs as soon as all the remnants of flake graphite have been neutralized. As the additions are increased further, a zone of increasing carbide stability and decreasing carbide metastability is obtained. The result of an addition to saturation produces a condition where the carbides are fully stable. In the conventional techniques for making nodular cast iron, the addition of a graphitizer to a melt containing stable carbides will only result in stable carbides in the final structure, together, with some very poor nodular graphite and even flake graphite. If a graphitizing addition is made when the melt consists only of metastable carbides, then all the graphite produced is in the nodular and spheroidal form. The basis of this invention is to add sumcient carbide metastabilizing agents to arrive at a condition in the melt which includes a substantial quantity of stable carbides. These stable carbides are then destabilized by the addition of the agents of this invention to the point where they may be graphitized by conventional means so as to provide a final structure with the graphite fully in the nodular form. The diagram represented by the drawing is only intended to establish a basis of principle. It cannot be taken too specifically, because the line of demarcation of the various zones may vary from melt to melt or according to the specific type of metastabilizing agent used. The wedge test is well known to those skilled in the art; however, the following explanation of the same will be given.

The Wedge test comprises the pouring of a casting of a predetermined length and of wedge in cross section with an acute angle of approximately 20 to 30 degrees. The wedges may be of several sizes, namely, one-half inch base with approximately 28.5 acute angle, threefourths inch base with approximately 26.75 acute angle, one inch base with approximately 25 acute angle, and two inch base with approximately 23.5 acute angle. After the wedge casting is poured and cooled, it is broken in two so that the carbide balance may be observed. Upon observation, one can discern that the acute angle portion has a whitish appearance While the remaining base portion has a grayish-white appearance. In the white portion, the carbon is generally in combined form, and in the grayish-white portion the carbon is generally in graphite form. The white portion is unmachinable. The width across the face of the wedge at the zone of demarcation between the white and grayish-white portions is an indication of the effect obtained by the carbide metastabilizer.

The knife edge of the wedge test piece cools quickly while the heavy section cools much more slowly, and the result is a varying texture in the casting, from the knife edge to the center, and often a shrink spot in the center. The wedge-shaped casting has an acute angle, defining a knife edge, ranging from approximately 20 to 30 degrees.

Thus, when a mixture is said to have a 20/32 carbideelfect Wedge, it means that the distance across the face of the wedge at the line of demarcation is approximately 20/32 of an inch and is, in simple terms, referred to as a 20/32 wedge. Because of the design of the wedges the zone 'of demarcation will be substantially the same regardless of which size wedge is used.

The following example will illustrate the relationship between wedge value and a numerical value times all White.

If the thickness of the casting to be made were 1 inch .and if the melt from which this casting were poured had -a wedge value of 64/32, then the melt would be two times all white.

In terms of wedge values, which are well known to those skilled in the art, the zone of instability with respect to carbides, that is the zone of unstable carbides or graphite, shown as extending from line A to line B, would be represented by a wedge value ranging from zero to a just mottled condition. Similarly, the zone of carbide metastability extending from line B to line C on the diagram would be represented by a wedge value ranging from mottled to 1 times white. Passing from the line C to line B, the carbides become increasingly stable and at some point represented by line D there are relatively few unstable carbides in the melt. This would be represented by a wedge value of about 2 times white. From the point represented by line D up to line E, more stable carbides occur and the line E might be represented by a wedge value of approximately 5 times white. In the practice of this invention, the addition of carbide metastabilizers to the melt is made in such amount that the melt passes into the zone falling between the line C and the line E, originally starting in the zone falling in the lines A and B. By going into the area falling close to line D, a melt condition where some stable carbides occur is assured. This puts the melt in a position where any stable carbides that are present may then be converted to the metastable condition by the carbide destabilizing addition of the present invention. The lines or zones expressed in the diagram may also be referred to in terms of carbide metastabilizer content of the melt. For example, in the case of an element such as magnesium, the line B would be represented by an amount of magnesium very close to .04%. The line C would be represented by an amount of magnesium very close to .06%. The line D would be represented by an amount of magnesium very close to -.09% It should be emphasized here that this magnesium content represents the total magnesium content of the melt which is the sum of that magnesium which is combined with other elements and that magnesium which may exist as free magnesium. Without limiting this invention to exact theory, it is thought that it is only the magnesium which is free in the meltthat is capable of producing the metastable condition in the melt. A very small quantity of free magnesium will produce this metastable condition. In conventional magnesium treated nodular irons, a fading effect often occurs. This happens due to natural oxidation and elimination of free magnesium from the melt and conversion of this free magnesium into the combined form giving a loss of carbide metastability. This is a very hazardous condition, because it is impossible to decide at any moment how much magnesium is available for metastability of the total magnesium content that is present. For this reason, in the practice of this invention, we prefer to use wedge tests which we feel are more positive and more indicative of a state of carbide metastability. If the diagram is taken to represent increasing additions of carbide metastabilizing agents as we pass from the point A along the horizontal line to the right, it is possible to superimpose a line XY passing from the left to the right and through the respective zones of the chart. For the first amount of addition carbides remain unstable, this is because the addition is used up by combining with the sulphur, oxygen, and other melt impurities. When these impurities have been used up, the passage through the zone of metastability com mences and proceeds at an extremely rapid rate. In terms of wedge values, the addition causes the metal to pass from the mottled to the several times white condition at an extremely rapid rate. Because this condition of carbide metastability occurs so quickly, it is diflicult to stop completely at the required point for producing a nodular form of graphite by subsequent graphitization. Further increasing additions will cause only a slow approach to the condition where the carbides are all stable. If the carbide metastabilizing agent additions are stopped and graphitization additions are made at a point along line XY, represented by L, the resultant casting will contain nothing but flake graphite. If graphitization additions are conducted from the point M, the resultant casting will contain the nodular form of graphite. If it is conducted from the point N, the final casting will contain some stable carbides, a considerable amount of flake graphite, and some poor nodular graphite. When a carbide destabilizing addition, as taught in this invention, is made from the point N prior to or simultaneously with a graphitization addition, it quickly brings the melt down to a condition represented by the point 0. Graphitization from the point 0, then gives the same final structure as graphitization from the point M, in other words, the destabilizing addition will remove any danger of obtaining stable carbides, flake graphite, or poor nodular graphite in the final structure. By making the additions heavy enough to arrive at the point N and then adding the destabilizing addition, it is possible to insure the optimum structure in the nodular iron at all times. There is no danger of carrying the process of nodularization too far, nor is there the danger of not carrying it far enough. From this description it is apparent that a carbide destabilizing agent should:

(1) Remove stable carbides produced by the addition of carbide metastabilizing agent.

(2) Maintain the carbide metastability necessary for nodularization.

(3) Provide a graphitizing effect.

We have found that the first parts of this requirement can be accomplished by using materials that tend to produce gas, which may neutralize or remove elements such as magnesium. I have found that sodium, potassium, calcium, or lithium carbonates or cyanides or cyanamides are effective in this regard. The same eflect may be obtained by injection of carbon dioxide or nitrogen gas into the melt.

The second part of the requirement for a carbide destabilizing agent is best accomplished by elements such as calcium, sodium, potassium, and zirconium which are in themselves metastabilizing in nature.

The third part of the requirement is accomplished by using graphitizers such as silicon, and aluminum.

The following examples clearly illustrate the process of this invention. A heat of cast iron was melted and a test piece was cast. The analysis was:

Percent Total carbon 3 .53 Silicon 1.72 Manganese 0.39 Sulphur 0.06

Percent Sodium carbonate 50 Calcium fluorid 10 Ferro silicon 15 Calcium silic n 25 The amount of this mixture used was one and onehalf percent. The slag produced on the melt was skimmed ofta portion was poured into a test bar, and the balance poured into castings. The test bar was analyzed and found to contain:

Percent Total carbon 3.41 Silicon 2.08 Sulphur .016 Manganese .38 Magnesium .032

The wedge value was 5/32. After the castings had been poured, a second test bar was cast from the metal remaining in the ladle, this was found to contain 030% magnesium.

Both test bars were checked for structure and propertiesthey showed all graphite to be in the nodular form and to be very small in nature, with a matrix consisting of approximately 50% pearlite and 50% ferite. The properties of the first bar were 82,000 pounds per square inch tensile with 10.5% elongation and the second bar was 81,500 pounds per square inch with a 10.0% elongation.

The approximate time elapsing between the casting of the first test bar and the second test bar was ten minutes.

All the castings made from this melt were singularly free from shrinkage and were X-ray sound and completely free from dross and pinhole defects normally obtained in the same casting when the special procedure of this invention was not used.

In a second example illustrating the working of this invention, we produced a melt having a retained magnesium content of 095%. We then added to a first por- This reduced the chill value considerably and provided a cast iron with all the graphite in the spheroidal form. The retained magnesium content of the melt, after graphitizing, was .02%. The wedge values were 5/ 32 in the original melt, 8/32 after the magnesium addition, 3/ 32 after the destabilizing addition. Castings poured from this graphitized melt were completely free of shrinkage and pinhole defects.

To a second portion of the melt containing 095% magnesium, We only added .3% of silicon as ferro silicon and cast similar castings. These castings were found to contain shrink defects, pinhole defects, free carbides, and relatively poor nodular graphite. The magnesium content of these castings was 085%.

In a third example, a suitable melt was treated with a cerium alloy to a wedge value of 2% times white. The carbide destabilizing addition reduced this wedge value to about A; of white. The cerium content of the melt Was reduced from .035 to 0.10% and the graphite was found to be substantially nodular in form.

In a further series of melts, several carbide destablizing mixtures were tested. In-each case the initial magnesium content of the melt ranged between .05 and 07%, whereas the magnesium content of the destabilized bath ranged between .02 and 0.35%. In each case the final product contained wholly nodular graphite, and castings poured were free from dross and pinhole defects. The mixtures used in these tests are:

Mixture No. 4

Sodium cyanide 30 Calcium cyanide 30 Zirconium silic n 40 Mixture No. 5

Lithium carbonate 20 Calcium fluoride 20 Calcium silicon 50 Sodium fluoride 10 The method of treating the melt with carbide metastabilizing agents prior to adding the carbide destabilizer of this invention is of no importance. Thus, magnesium, for example, may be added as pure magnesium or as any of the well-known alloys of magnesium. It may be added to the surface of the metal, or it may be added under the surface by plunging or by injection. The essential part of the first step of this invention is to introduce sufiicient carbide metastabilizing agents to the melt to insure a completely nodular graphite. We prefer, in the case of magnesium, to have a residual magnesium content in the melt of .06% or more after this first step. In the second, or carbide destabilizing step, this magnesium is reduced to a value of under 04% usually to about 030% or .035 but at times as high as .045 In any case, the magnesium content of the final melt before casting is substantially lower than that of the melt before the carbide destabilizing addition is made. We prefer to limit the retained magnesium of the melt as the time of casting to a value of under 04%.

On occasion, we have reduced the magnesium content of the melt to a value as low as 010% without affecting the proportion of nodular or spheroidal graphite.

The amount of carbide destabilizer added to the melt does not appear to be critical. Where carbide metastabilizing additions have been heavy, it is wise to add a correspondingly heavy amount of carbide destabilizer. For instance, where the magnesium content of the melt is 08% or more, we prefer to use between 1 and 3% of the carbide destabilizer; where the magnesium content of the melt is of the order or" 04%, an addition of about /2% of carbide destabilizer appears to be effective.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in details may be resorted to without 7 departing from the'spirit and scope of the invention hereinafter claimed.

What is claimed is:

1. The process of producing a cast iron from a melt comprising the steps of adding an amount of a carbide metastabilizing agent to the melt so as to provide a substantially over-saturated carbide-iron solution containing metastable and stable carbides, and adding an agent selected from the group consisting of the carbonates, cyanides, and cyanamides of sodium, potassium, calcium, and lithium; carbon dioxide gas; and nitrogen gas; to the melt to change said stable carbides to metastable carbides, and graphitizing the melt.

2. The process of producing a nodular cast iron from a melt comprising the steps of adding an amount of magnesium to the melt so as to provide a substantially oversaturated carbide iron solution containing metastable and stable carbides in the range of from one times to five times White, adding an agent to the melt to reduce the amount of magnesium in the melt so as to increase carbide metastability and provide a melt in the range of from mottled ,to one times white, and graphitizing the melt.

3. The process of producing a nodular cast iron from a melt comprising the steps of adding an amount of nodule impelling agent to the melt sons to provide a substantially oversaturated carbide iron solution containing metastable and stable carbides in the range of from one time to five times white, adding an agent to the melt to reduce the amount of nodule impelling agent in the melt so as to increase carbide metastability and provide a melt in the range of from mottled to one times white, and graphitizing the melt.

4. The process of producing a nodular cast iron from a metal comprising the steps of adding an amount of magnesium to the melt so as to provide a substantially oversaturated carbide iron solution containing metastable and stable carbides in the range of from one times to five times White, adding an agent to the melt to reduce the magnesium content of the melt substantially so as to increase carbide metastability in the melt and graphitizing the melt.

5. The process of producing a nodular cast iron from a melt comprisingthe steps of adding an amount of nodule impelling agent to the melt so as to provide a substantially oversaturated carbide iron solution containing metastable and stable carbides in the .range of from one times to five times White, adding an agent to the melt to reduce the nodule impelling agent content of the melt substantially so as to increase carbide metastability in the melt and graphitizing the melt.

6. The process of producing a cast iron from a melt, comprising the steps of, adding an amount of magnesium to the melt so as to provide a substantially over-saturated carbide iron solution containing metastable and stable carbides, adding an agent to the melt to reduce the amount of magnesium in the melt and to change stable carbides to metastable carbides, and graphitizing the melt to change metastable carbides to the nodular form of graphite.

7. The process of producing a cast iron from a melt, comprising the steps of, adding an amount of a nodule impelling agent to the melt so as to provide a substantially over-saturated carbide iron solution containing metastable and stable carbides, adding an agent to the melt to reduce the amount of nodule impelling agent in the melt and to change stable carbides to metastable carbides, and graphitizing the melt to change metastable carbides to the nodular form of graphite.

References .Cited in the file of this patent UNITED STATES PATENTS 1,683,087 Meehan Sept. 4, 1928 2,038,639 Burgess Apr. 28, 1936 2,527,037 Smalley Oct. 24, 1950 2,747,990 Morrogh May 29, 1956 2,821,473 Moore Ian. 28, 1958 OTHER REFERENCES Millis et 2.1.: 2,485,760, patent file, page 32, Table IX. 

2. THE PROCESS OF PRODUCING A NODULAR CAST IRON FROM A MELT COMPRISING THE STEPS OF ADDING AN AMOUNT OF MAGNESIUM TO THE MELT SO AS TO PROVIDE A SUBSTANTIALLY OVERSATURATED CARBIDE IRON SOLUTION CONTAINING METASTABLE AND STABLE CARBIDES IN THE RANGE OF FROM ONE TIMES TO FIVE TIMES WHITE, ADDING AN AGENT TO THE MELT TO REDUCE THE AMOUNT OF MAGNESIUM IN THE MELT SO AS TO INCREASE CARBIDE METASTABILITY AND PROVIDE A MELT IN THE RANGE OF FROM MOTTLED TO ONE TIMES WHITE, AND GRAPHITIZING THE MELT. 